As a method replacing a conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor was proposed and has been practically employed. The method employs a radiation image storage panel comprising a support and a stimulable phosphor layer (stimulable phosphor sheet) provided thereon, and comprises the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The panel thus treated is subjected to a step for erasing a radiation image remaining therein, and then stored for the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly employed.
In the radiation image recording and reproducing method, a radiation image is obtained with a sufficient amount of information by applying a radiation to an object at a considerably smaller dose, as compared with a conventional radiography using a combination of a radiographic film and radiographic intensifying screen. Further, the method is very advantageous from the viewpoints of conservation of resource and economic efficiency because the radiation image storage panel can be repeatedly used while the radiographic film is consumed for each radiographic process in the conventional radiography.
The stimulable phosphor produces stimulated emission when excited with stimulating rays after having been exposed to a radiation, and a practically used one gives a stimulated emission of a wavelength in the range of 300 to 500 nm when irradiated with stimulating rays of a wavelength in the range of 400 to 900 nm. A typical example of that phosphor is a rare earth activated alkaline earth metal halide phosphor.
The radiation image storage panel has a basic structure comprising a support and a stimulable phosphor layer provided on one surface of said support. If the phosphor layer is self-supporting, the support may be omitted. The phosphor layer usually comprises a binder and stimulable phosphor particles dispersed therein, but it may consist of agglomerated phosphor without binder. The phosphor layer containing no binder can be formed by deposition process or firing process. Further, the layer comprising agglomerated phosphor soaked with a polymer is also known. In any of these phosphor layers, the stimulable phosphor produces stimulated emission when excited with a stimulating ray after having been exposed to a radiation such as X-ray. The radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and accordingly a radiation image of the object is produced in the panel in the from of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the panel with stimulating rays, and the stimulated emission is then photoelectrically detected to obtain electric signals, so as to reproduce a visible image from the obtained signals.
In general, a transparent protective film of polymer material is placed on the free surface (surface not facing the support) of the phosphor layer to keep the layer from chemical deterioration or physical damage.
The storage panel comprising the stimulable phosphor of rare earth activated alkaline earth metal halide has good sensitivity and gives an image of high sharpness, and hence that phosphor is practically advantageous. However, since the radiation image recording and reproducing method is getting more and more widely used, it is further desired to develop a stimulable phosphor having further improved characteristics.
Among the known rare earth activated alkaline earth metal halide stimulable phosphors, cerium activated phosphors are preferred because they give a rapid response. Accordingly, from a panel comprising the cerium activated phosphor, the image information can be rapidly read out.
Japanese Patent Provisional Publications No. 55-84389 and No. 7-126617 disclose a cerium activated alkaline earth metal fluorohalide stimulable phosphor which is improved by incorporating additives of a trivalent metal element (such as In) or a divalent metal element (such as Sn).